Oil compensation system for electric power cables impregnated with insulating oil

ABSTRACT

A system for compensating for normal and abnormal changes in oil volume in an oil-impregnated electric power cable comprising one or more two-part pressure tanks with one part containing the oil and connected to the cable oil duct to supply such oil thereto and with the other part applying pressure to the oil in the one part over the normal operating oil pressure range and one or more similar two-part pressure tanks similarly connected to the cable oil duct through a flow regulating valve but having the oil under a pressure at or below the lowest normal operating oil pressure so that oil is supplied to the cable duct thereby only when the cable oil pressure decreases to, or below, the lowest normal operating pressure. unit is mounted in an opening in the bottom of a cup-shaped plastic jacket, and has two terminals which project into the jacket and which have lead wires connected thereto. The wires extend out through notched openings in the side of the jacket, and are retained in those openings by a ring member which seats on the open end of the jacket and which has a diametral web at opposite sides of which the lead wires are disposed. The can has a flared open end; and the can and ring member are secured in place by epoxy flowed into the open end of the can and around its flared end and into the jacket to a point beneath the notched openings.

United States Patent [191 Ferrentino June 19, 1973 1 OIL COMPENSATIONSYSTEM FOR ELECTRIC POWER CABLES IMPREGNATED WITH INSULATING OIL [75]Inventor: Antonio Ferrentino, Monza, Italy [73] Assignee: IndustriePirelli Societa Per Azioni,

Milan, Italy 22 Filed: Apr. 7, 1972 211 Appl. No.: 242,127

[30] Foreign Application Priority Data June 11, 1971 Italy 25700 A/7l[52] US. Cl 137/255, 29/622, 174/52 PE,

7 200/168 G [51] Int. Cl..' F17d 1/00 [58] Field of Search 137/255, 265,208;

. 174/52 PE; 29/622; 200/168 G [56] References Cited UNITED STATESPATENTS 3,158,193 11/1964 Anderson 137/255 3,584,106 6/1971 Miller174/52 PE 3,636,622 1/1970 Schmitt 29/622 Primary Examiner-Henry T.Klinksiek Assistant ExaminerRobert J. Miller Attorney-Lorimer P. Brooks,Alfred L. Haffner, Jr., I-Iarold Haidt et al.

720 Beer Make 75%:

[57] ABSTRACT A system for compensating for normal and abnormal changesin oil volume in an oil-impregnated electric power cable comprising oneor more two-part pressure tanks with one part containing the oil andconnected to the cable oil duct to supply such oil thereto and with theother part applying pressure to the oil in the one part over the normaloperating oil pressure range and one or more similar two-part pressuretanks similarly connected to the cable oil duct through a flowregulating valve but having the oil under a pressure at or below thelowest normal operating oil pressure so that oil is supplied to thecable duct thereby only when the cable oil pressure decreases to, orbelow, the lowest normal operating pressure. unit is mounted in anopening in the bottom of a cup-shaped plastic jacket, and has twoterminals which project into the jacket and which have lead wiresconnected thereto. The wires extend out through notched openings in theside of the jacket, and

are retained in those openings by a ring member which seats on the openend of the jacket and which has a diametral web at opposite sides ofwhich the lead wires are disposed. The can has a flared open end; andthe can and ring member are secured in place by epoxy flowed into theopen end of the can and around its flared end and into the jacket to apoint beneath the notched openings.

9 Claims, 3 Drawing Figures 7170 39 97- fimssue: Ewes Patented June 19,1973 3 Sheets-Shut 2 Patented June 19, 1973 3 Sheets-Sheet 3 OILCOMPENSATION SYSTEM FOR ELECTRIC POWER CABLES IMPREGNATED WITHINSULATING OIL The present invention concerns an improved system whichcan be used particularly, but not exclusively, for compensating for theloss of the impregnating fluid in power cables filled with oil at highpressure (above atmospheres) especially long-length cables, such assubmarine cables, in the event of leaks, and which provides automaticaction, without the use of moving mechanical elements, or apparatus formaintaining the oil at a pre-established pressure and for apredetermined time.

The expression compensation system means the group of means which actsautomatically in order to supply insulating oil to an oil-filled cablein the case of a leak, or to absorb if the oil volume should vary as aconsequence of the temperature variations.

Up to this time, the oil pressure in a leaking cable, in particular, incables operating under high pressures, was maintained by means of pumpswhich, although they can provide an automatic action in case of leaks,have serious disadvantages.

Among these, the following are the most relevant: the pumps are oftenunable to ensure a good tightness, the insulating fluid can becontaminated, the moving mechanical elements require a continuous watch,and operating mechanisms are necessary, such as pressure switches,relays, etc., which make the system complex and delicate. I

The present invention aims at providing a compensation system,comprising a device for automatically maintaining the oil pressure inthe event of leaks, which, by eliminating the presence of rotatingelements and reducing to a minimum the control means, obviates theabove-mentioned disadvantages.

In particular, the system according to the invention provides themaximum protection against contamination, is structurally simple and,being based on essentially fixed parts, requires a minimum of watchingand maintenance as compared to the conventional systems, therebyproviding, in addition to such technical advantages, remarkable economicadvantages.

More particularly, the object of the present invention is an improvedcompensation system for power cables impregnated with insulating oil,which comprises a first group of one or more pressure tanks, with arelated expansion chamber, able to operate at high pressures andarranged in parallel to one another with respect to the cable, and withtheir thrust oil side at operating pressure, and at least a second groupof one or more pressure tanks, with a related expansion chamber,arranged in parallel to one another and to the first group on the sameend of the cable, the pressure tanks of the second group being of thetype able to operate at high pressure and having their thrust oil side"rated at a pressure lower than the operating pressure of the cable. Thepressure tanks are of a known type and have means for supplying andabsorbing the cable fluid, preferably insulating oil, which is fullyseparated from the thrust oil.

In the compensating pressure tanks, the separating elastic diaphragmdefines two sides, one constituted by the space occupied bytheinsulating oil, which will be called insulating oil side", and theother constituted by the oil having the task of exerting a pressure onthe insulating oil through the elastic diaphragm, which will be calledthrust oil side. The elastic diaphragm moves between two stop positions:one (upper stop position) corresponds to the maximum volume ofinsulating oil which can be contained in the pressure tanks, while theother (lower stop position) corresponds to the complete exhaustion ofsaid insulating oil which can be contained in the pressure tanks.

The accompanying sheets of drawing illustrate by way of exampleembodiments of the invention, and in such drawings:

FIG. 1 represents diagrammatically a preferred embodiment of thecompensation system according to the invention;

FIG. 2 represents diagrammatically a further embodiment of thecompensation system according to the invention; and

FIG. 3 is a graph illustrating the operation of the compensation systemfor the cable as a function of the pressure existing in the latter.

The preferred embodiment of the compensation system according to theinvention and which is shown in FIG. 1, comprises at least a first group10 of pressure tanks 10' of the type able to operate athigh pressures.Pressures over 10 atmospheres are considered as high, but the system canalso be used for lower pressures.

The tanks 10 are arranged in parallel to one another and are connected,at their thrust oil side, through a pipe 11, to a single expansionchamber 12 provided with a filling and discharge valve 13 and with anadmission valve 14. Although not preferred, a single tank 10' instead ofa group of tanks 10' could be used.

At the insulating oil side, the group 10 of parallel pressure tanks 10is connected, through a pipe 15 provided with a valve 28, to themanifold 16, which extends to the oil duct of the cable C to becompensated and to one end of the latter. In other words, the tanks 10are connected in parallel to one another and to the cable C.

The pipe 15 is also joined to a pipe 17 coming from the insulating oilside of a second group 18 of pressure tanks 18 arranged in parallel. Ifdesired, the pipe 17 may also connect, via pipe 17a, to a third group 19of pressure tanks 19. The pipe 17 is provided with a flowregulatingvalve 26 and with an alarm device which comprises a pressure switch 27and an alarm 27a. Instead ofa group of tanks 18' and 1?. a single tank18' and a single tank 19' may be used.

The pressure tanks 18' and 19, forming groups 18 and 19, are all equaland of the type which can operate at high pressures. The groups 18 and19 of tanks are therefore arranged in parallel to the first group 10 onthe same end of cable C.

The tanks 10' and the tanks 18 and 19' are preferably those described inmy co-pending application Ser. No. 146,295 filed May 24, 1971, now US.Pat. No. 3,695,297 and entitled Compensating. Pressure Tank forOil-Filled Cables. As described therein, each tank is divided into twochambers by an elastic diaphragm or bellows which is limited topredetermined travel by a pair of stops. One of the chambers is filledwith oil of I 23, provided with a filling and discharge valve 24 andwith an admission valve 25.

The pipes 15 and 17 are provided with valves 28 and 29, respectively,which serve for filling the system with insulating oil at the time ofits fitting and installation. In fact, at the same time, insulating oilis supplied to the pressure tanks 18' and '19, previously filled onlywith the thrust oil at the thrust oil side, through said valves 28 and29, while the discharge valves 13 and 24 of chambers 12 and 23 are open.

During the filling with insulating oil, the thrust oil passes fromgroups 10, 18 and 19 of pressure tanks in parallel to the expansionchambers 12 and 23, respectively. In each of these chambers, the volumeof the thrust oil must be at least equal to, or greater than, the oilvolume corresponding to the total capacity of the group or groups oftanks connected thereto, and can be adjusted by means of the filling anddischarge valves 13 and 24.

After having closed the supply valves 28 and 29,

compressed air or gas is admitted through the admission valves 14 and25, so as to establish in chamber 12 a pressure p corresponding to thecable oil operating pressure, and in chamber 23 a predetermined pressurep lower than the cable oil operating pressure, taking also into accountits possible variations with temperature. For example, with a tolerancerange of: 10 percent p, p should be no greater than p minus 10 percentp.

As long as no leaks occur in the cable, the group 10 of pressure tanks10' compensate for the variations in the volume of insulating oil incable C which are due to the temperature variations in the cable andwhich are within the tolerance limits indicated for that type of cable.

In these conditions, the groups 18 and 19 of pressure tanks 18 and 19',respectively, cannot enter into compensating action, since pressure p onthe insulating oil is higher than pressure p in the expansion chamber ofthe thrust oil, and therefore, the separation diaphragms between theinsulating oil side and the thrust oil side are all brought by theinsulating oil to the stop position corresponding to the maximum volumeof insulating oil consistent with such pressure tanks (upper st'opposition).

The operation of the system is better understood from the graph of FIG.3, in which the abscissae represent the oil volumes of the systemexpressed, for example, in cubic meters, and the ordinates represent thepressures in the manifold 16 expressed, for example, in kilograms persquare centimeter.

The origin of the abscissae is made coincident with the initialcondition of maximum volume of insulating oil which can be contained inthe system, namely, when the diaphragms in the pressure tanks havereached the upper stop position and a puressure 2 exists in the manifold16.

Assuming that, for a temperature variation (decrease) or for a leak, theoil flows from the pressure tanks 10 into the cable C, a pressure dropwill be noticed, in this case, as a result of the flow of oil from thecompensating system towards the cable. If the volumepressure curve(assuming, for present purposes, isothermal conditions) corresponding tothe variations in question is examined, a curve AB will be obtained,which has a hyperbolic shape.

The point B corresponds to the complete exhaustion of the insulating oilwhich can be supplied by the group 10 of pressure tanks 10, since thediaphragms have reached the other stop position (lower stop position).The points A and B correspond theoretically to the limits within whichare contained the admissible variations of the volumes of insulating oilin the cable. Obviously, in the practice, such variations will becontained in a zone which has an adequate margin of safety with respectto the extreme points. Therefore, the point B is actually reached onlyin the event of leaks.

From the point B a further amount of oil flows out of the cable throughthe leak only as a result of the elasticity of the oil ducts or pipes(cable included). Therefore, the pressure would quickly drop to zero atthe point F if the tanks 18 and 19 were not connected to the system.However, at the point C, corresponding to the operating pressure p ofthe pressure tanks 18' and 19, these tanks enter into the compensatingaction and supply insulating oil to the cable C.

The curve of pressures, starting from the point C, follows the curve CD.Again, assuming isothermal conditions, the curve CD has a hyperbolicshape up to the point D, corresponding to the lower stop positions ofthe pressure tanks 18 and 19'. If the cable is not repaired before thepressure reaches the point D, pressure falls finally to zero along theline DE, in which oil leaks from the cable only because of theelasticity of the ducts.

The flow-regulating valve 26 ensures that during the operation of groups18 and 19 of tanks the insulating oil is not rapidly dispersed out ofcable C, Le. it supplies said cable C at a regulated and constantdelivery rate, so as to prevent both the quick exhaustion of the reserveoil and the penetration of air in ground cables, or of water insubmarine cables, at the point of the leak. Moreover, the volume ofgroups 18 and 19 of tanks may be of a size which will maintain theemergency supply even for several days, so as to allow the personnel towork on the cable beginning at the time of the operation of the warningalarm 27a, which is operated by the pressure switch 27 when the pressuredecreases below the limits for normal operations, e.g., the point B inFIG. 3.

If necessary, the groups of tanks 18 and 19 can be repeatedly filledthrough the valve 29 even during the emergency phase.

The further preferred embodiment of the system, illustrated in FIG. 2(in which elements like those of FIG. 1 are indicated with likereference numerals), differs from the above described embodiment in thatthe two expansion chambers 12 and 23, instead of being completelyseparate, are connected through their respective upper parts intended tocontain gas under pressure, namely, through those parts which are abovethe respective uppermost possible levels of the thrust oil, by means ofa pipe 30 carrying a pressure-reducing valve 31.

During the installation of the system, it is therefore sufficient toadjust chamber 12 to operating pressure p and by means of the reducingvalve 31 the pressure in chamber 23 is automatically brought to 2. p.

This system requires a chamber 23 of a smaller volume with respect tothe system shown in FIG. 1, and is therefore preferred when only reducedroom is available.

On the other hand, the system of FIG. 1 is reliable in course of time,owing to its structural simplicity.

It will be understood by those skilled in the art that the details ofconstruction of the invention can of course be varied and that variousmodifications may be made therein without departing from the principlesof the invention.

What is claimed is:

1. Apparatus for compensating for changes in oil volume in an oilimpregnated electric power cable having means for supplying said oilthereto, said oil normally being maintained under a pressure within apredetermined range, said apparatus comprising at least one firstpressure tank having a chamber for containing said oil and means formaintaining said oil under pressure within said predetermined range, atleast one second pressure tank having a chamber for containing said oiland means for maintaining said oil in said lastmentioned chamber under apressure substantially at least as low as the lowest pressure in saidpredetermined range, and means for connecting said first and secondtanks to said means for supplying oil to said cable for oil flowtherebetween.

2. Apparatus as set forth in claim 1, further comprising at least onefurther tank similar to said first tank and similarly having means formaintaining the oil in the chamber thereof under a pressure within saidpredetermined range connected in parallel with said first tank and atleast one further tank similar to said second tank and similarly havingmeans for maintaining the oil in the chamber thereof under a pressuresubstantially at least as low as the lowest pressure in saidpredetermined range connected in parallel with said second tank.

3. Apparatus as set forth in claim 1, wherein each of said first andsecond tanks comprises an elastic wall dividing at le'ast a portion ofthe tank into said chamber thereof and a further chamber and said meansfor maintaining the oil in said first-mentioned chamber under pressurecomprises means for supplying fluid under pressure to said furtherchamber.

4. Apparatus as set forth in claim 3, wherein said means for supplyingfluid under pressure comprises a pair of expansion chambers, oneconnected to the further chamber of said first tank and one connected tothe further chamber of said second tank.

5. Apparatus as set forth in claim 4, further comprising means includinga pressure reducing valve interconnecting said further chambers, saidreducing valve reducing the pressure in the further chamber of saidsecond tank with respect to the pressure in the further chamber of saidfirst tank.

6. Apparatus as set forth in claim 3, further comprising means limitingthe movement of said elastic wall to within a predetermined range ofmovement.

7. Apparatus as set forth in claim 1, wherein said means for connectingsaid first and second tanks to said means for supplying oil to saidcable comprises a flow regulating valve connected between said secondtank and said means for supplying oil to said cable for limiting therate of flow of oil from said second tank to said cable.

8. Apparatus as set forth in claim 1, further comprising meansresponsive to the drop of oil pressure below a predetermined valueconnected to said means for connecting said first and second tanks tosaid means for supplying oil to said cable.

9. Apparatus as set forth in claim 8, further comprising alarm meansconnected to said means responsive to the drop of oil pressure andoperable thereby when said pressure drops below a predetermined value.

1. ApparatuS for compensating for changes in oil volume in an oilimpregnated electric power cable having means for supplying said oilthereto, said oil normally being maintained under a pressure within apredetermined range, said apparatus comprising at least one firstpressure tank having a chamber for containing said oil and means formaintaining said oil under pressure within said predetermined range, atleast one second pressure tank having a chamber for containing said oiland means for maintaining said oil in said last-mentioned chamber undera pressure substantially at least as low as the lowest pressure in saidpredetermined range, and means for connecting said first and secondtanks to said means for supplying oil to said cable for oil flowtherebetween.
 2. Apparatus as set forth in claim 1, further comprisingat least one further tank similar to said first tank and similarlyhaving means for maintaining the oil in the chamber thereof under apressure within said predetermined range connected in parallel with saidfirst tank and at least one further tank similar to said second tank andsimilarly having means for maintaining the oil in the chamber thereofunder a pressure substantially at least as low as the lowest pressure insaid predetermined range connected in parallel with said second tank. 3.Apparatus as set forth in claim 1, wherein each of said first and secondtanks comprises an elastic wall dividing at least a portion of the tankinto said chamber thereof and a further chamber and said means formaintaining the oil in said first-mentioned chamber under pressurecomprises means for supplying fluid under pressure to said furtherchamber.
 4. Apparatus as set forth in claim 3, wherein said means forsupplying fluid under pressure comprises a pair of expansion chambers,one connected to the further chamber of said first tank and oneconnected to the further chamber of said second tank.
 5. Apparatus asset forth in claim 4, further comprising means including a pressurereducing valve interconnecting said further chambers, said reducingvalve reducing the pressure in the further chamber of said second tankwith respect to the pressure in the further chamber of said first tank.6. Apparatus as set forth in claim 3, further comprising means limitingthe movement of said elastic wall to within a predetermined range ofmovement.
 7. Apparatus as set forth in claim 1, wherein said means forconnecting said first and second tanks to said means for supplying oilto said cable comprises a flow regulating valve connected between saidsecond tank and said means for supplying oil to said cable for limitingthe rate of flow of oil from said second tank to said cable. 8.Apparatus as set forth in claim 1, further comprising means responsiveto the drop of oil pressure below a predetermined value connected tosaid means for connecting said first and second tanks to said means forsupplying oil to said cable.
 9. Apparatus as set forth in claim 8,further comprising alarm means connected to said means responsive to thedrop of oil pressure and operable thereby when said pressure drops belowa predetermined value.